"I run windows Vista, it cost me £200 .. do I upgrade to win 7 for probably about £100, or pay what's likely to be about £200"
Since you still run Vista, I'll make a few assumptions and guess your PC is just an appliance/tool, rather than used for gaming or a hobby. So your PC is working but possible showing its age?
If that is the case, wait for Windows 9, then buy yourself a new PC with an OEM version of Windows installed! You'll be able to pick up a reasonable system for £300-400.